String concatenation using LINQ to create a CSV/PSV string

I rather like IEnumerable<T> than IList<T>, parameters should be as general as posible. I think it's really getting very fun up here. Here's my choice, just for strings.

public static string Join(this IEnumerable<string> parts, string separator)
{
    if (! parts.Any())
        return string.Empty;

    var builder = new StringBuilder();

    builder.Append(parts.First()) ;

    foreach(var part in parts.Skip(1))
    {
        builder.Append(separator) ;
        builder.Append(part);
    }

    return builder.ToString();
}